Semiconductor chips are in widespread use in numerous technical applications. However, they are still very complex and expensive to produce. Although silicon substrates can be thinned to very low layer thicknesses, so that they become flexible, these methods are likewise very expensive, and consequently flexible or curved microchips are suitable only for demanding applications for which increased costs are acceptable. The use of organic semiconductors offers the possibility of inexpensive production of microelectronic semiconductor circuits on flexible substrates. One application is, for example, a thin film with integrated control elements for liquid crystal displays. A further application area is transponder technology, where, for example, information on a product is stored on what are known as tags.
Field-effect transistors are used as switches in electronic circuits. In this context, a semiconductor which is arranged between a source electrode and a drain electrode, each composed of an electrically conductive material, in the off state of the transistor in each case acts as an insulator, whereas in the on state of the transistor, under the influence of the field of a gate electrode, a charge carrier channel is formed. In this state, electrical charge carriers are injected into the semiconductor layer at the source contact and extracted from the semiconductor layer at the drain contact, so that an electric current flows through the semiconductor layer or through the charge channel generated in the semiconductor layer from source to drain.
On account of the different Fermi levels of semiconductor material and contact material, an asymmetric diffusion process of the charge carriers occurs at the contact surface between the two materials. The different energy of the Fermi levels of the two materials means that there is an energy difference which is compensated for by the transfer of charge carriers. Consequently, an interfacial potential is built up, which counteracts the transfer of charge carriers between the two layers when an external potential difference is applied. The result is the formation of a potential barrier which the charge carriers have to overcome when they enter the semiconductor material from the electrically conductive contact or leave the semiconductor material to pass into the electrically conductive contact. The higher or wider the potential barrier, the lower the tunneling current which is formed as a result of the charge carriers tunneling through the potential barrier. A low tunneling current corresponds to a high contact resistance. In the case of semiconductor components based on inorganic semiconductors, an increase in the contact resistance is combated by doping the inorganic semiconductor in a boundary layer oriented toward the contact surface. The doping changes the energy of the Fermi level in the inorganic semiconductor, i.e., the difference between the Fermi levels of contact material and semiconductor material is reduced. Consequently, either the potential barrier is reduced, allowing a significantly greater number of charge carriers to overcome the potential barrier and flood the opposite material, or the potential barrier is narrowed, with the result that the probability of charge carriers tunneling through the potential barrier is increased. In both cases, the contact resistance is reduced.
When fabricating field-effect transistors based on amorphous or polycrystalline silicon layers, the doping of the contact regions is effected by the introduction of phosphorus or boron into the silicon layer in the vicinity of the source and drain contacts. The phosphorus or boron atoms are incorporated in the silicon network and act as charge donors or charge acceptors, with the result that the density of free charge carriers and therefore the electrical conductivity of the silicon increases in the doped region. This reduces the difference between the Fermi levels of the contact material and the doped semiconductor material. The doping substance is introduced into the silicon only in the region of the source and drain contacts, but not in the channel region, in which a charge carrier channel is formed under the influence of the field of the gate electrode. Since phosphorus and boron form covalent bonds with the silicon, there is no risk of these atoms diffusing into the channel region, and consequently a low electrical conductivity continues to be ensured in the channel region.
If the doping of the contact regions is sufficiently high, the tunneling probability, even in the off state, is so high that the junction between the contact material and the inorganic semiconductor material loses its blocking capacity and acquires good conductivity in both directions.
Field-effect transistors based on organic semiconductors are of interest for a wide range of electronic applications which require extremely low manufacturing costs, flexible or unbreakable substrates, or the fabrication of transistors and integrated circuits over large active areas. By way of example, organic field-effect transistors are suitable as pixel control elements in active matrix displays. Displays of this type are usually produced using field-effect transistors based on amorphous or polycrystalline silicon layers. The temperatures of usually more than 250° C. which are necessary for the fabrication of high-quality transistors based on amorphous or polycrystalline silicon layers necessitate the use of rigid and therefore breakable glass or quartz substrates. On account of the relatively low temperatures at which transistors based on organic semiconductors are fabricated, normally of less than 100° C., organic transistors allow the production of active matrix displays using inexpensive, flexible, transparent, unbreakable polymer sheets with considerable advantages over glass or quartz substrates.
A further application area for organic field-effect transistors is the production of very inexpensive integrated circuits as are used, for example, for the active marking and identification of goods and products. These transponders, as they are known, are usually produced using integrated circuits based on single-crystal silicon, which leads to considerable costs with regard to the build-up and connection technology. The production of transponders based on organic transistors would lead to huge cost reductions and could help transponder technology to make a worldwide breakthrough.
One of the main problems of using organic field-effect transistors is the relatively poor electrical properties of the source and drain contacts, i.e. the high contact resistances thereof. The source and drain contacts of organic transistors are usually produced using inorganic metals or with the aid of conductive polymers, in order thereby to ensure the highest possible electrical conductivity of the contacts. Most organic semiconductors which are suitable for use in organic field-effect transistors have very low electrical conductivities. For example, pentacene, which is frequently used for the fabrication of organic field-effect transistors, has a very low electrical conductivity of 10−14 Ω−1 cm−1. If the organic semiconductor has a low electrical conductivity, therefore, there is a considerable difference between the Fermi levels of electrically conductive contact material and organic semiconductor material at the contact surface. This leads to the formation of a high potential barrier with a low tunneling probability for the transfer of charge carriers. Consequently, source and drain contacts often have high contact resistances, which means that high electric field strengths are required at the contacts in order to inject and extract charge carriers. Therefore, it is not the conductivity of the contact itself which is a limiting factor, but rather the low conductivity of the semiconductor regions adjoining the contacts, into or from which the charge carriers are injected or extracted.
To improve the electrical properties of the source and drain contacts, therefore, a high electrical conductivity of the organic semiconductor is desirable in the regions adjoining the contacts, in order to reduce the difference in the Fermi levels between organic semiconductor and contact material and thereby to lower the contact resistances. On the other hand, a high electrical conductivity of the organic semiconductor in the channel region has an adverse effect on the properties of the transistor. A significant electrical conductivity in the channel region inevitably leads to high leakage currents, i.e. to relatively high electric current intensities in the off state of the field-effect transistor. For many applications, however, low leakage currents in the range from 10−12 A or below are imperative. Moreover, a high electrical conductivity leads to the ratio between the maximum on current and minimum off current being too low. Many applications require the highest possible ratio between on current and off current, in the range of 107 or above, since this ratio reflects the modulation performance and the gain of the transistor. In the channel region, therefore, a low electrical conductivity of the organic semiconductor is required, whereas in the region of the source and drain contacts a high electrical conductivity is required in order to improve the contact properties between organic semiconductor material and the material of the contacts.
The electrical conductivity of many organic semiconductors, like that of inorganic semiconductors, can be increased by the introduction of suitable doping substances. However, there are problems with achieving positional selectivity during doping. The doping substances in the organic semiconductors are not bonded to one specific position and can move freely within the material. Even if the doping process can originally be restricted to a certain region, for example the regions around the source and drain contacts, the doping substances subsequently migrate through the entire organic semiconductor layer, in particular under the influence of the electric field applied between the source contact and drain contact in order to operate the transistor. The diffusion of the doping substance within the organic semiconductor layer inevitably increases the electrical conductivity in the channel region.
I. Kymissis, C.D. Dimitrakopoulos and S. Purushothaman, “High-Performance Bottom Electrode Organic Thin-Film Transistors” IEEE Transactions on Electron Devices, Vol. 48, No. 6, Jun. 2001, p. 1060–1061, describe a semiconductor device with a reduced contact resistance, in which first of all a monomolecular layer of 1-hexadecanethiol is applied to chromium/gold electrodes, and then a layer of pentacene as organic semiconductor material is applied to the layer of 1-hexadecanethiol. This arrangement makes it possible to significantly lower the contact resistance for the transfer of charge carriers between electrode and semiconductor section. The 1-hexadecanethiol molecules arranged at the interface between contact and organic semiconductor act as charge transfer molecules. They are in direct contact both with the contact material and with the organic semiconductor layer. On account of their molecular structure, the charge transfer molecules can impose a transfer of charge carriers between the contact material, in which there is an excess of charge carriers, and the organic semiconductor layer, in which there is a deficit of charge carriers. In this way, an excess of charge carriers in the organic semiconductor layer can be produced in the region of the source and drain contacts, with the result that the contact resistance is significantly reduced. The thiol groups of the 1-hexadecanethiol form a covalent bond with the surface of the gold contacts, which effects local fixing of the molecules. Therefore, even under the influence of a field applied between source electrode and drain electrode, the charge transfer molecules do not migrate into those portions of the organic semiconductor section in which the channel region is formed.
However, gold has the drawback that it generally bonds very poorly to inorganic layers, such as for example silicon dioxide. To improve the bonding of the gold contacts, therefore, a thin film of chromium or titanium as bonding agent is often applied directly prior to the deposition of the gold layer. However, this has the drawback of making the patterning of the metal layer required for the production of the contact structures more difficult. Furthermore, thiols are also unsuitable as charge transfer molecules since it is not possible to achieve a sufficient bonding strength with all metals suitable for the production of contacts to prevent the thiols from diffusing out of the boundary layer between contact and semiconductor material.